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Balancing a System

Piping systems providing heat transfer fluid to a variety of loads can present their own special problem s. Since there aremultiple paths for the fluid to travel, the system naturally balances so the press ure drop around each loop is equal. It is

highly unlikely that when all of the valves are fully open, the flow rate will equal the design flow rate needed to provide the

necess ary heating or cooling to the various loads. As a result the system must be balanced us ing flow control valves or

orifices.

If theheating or cooling system is not balanced, som e loads will get excess ive flow and other loads will be starved for flow.

This results in the system not operating as designed. Anytime a looped heat transfer system is built or expanded, the

system s hould be balanced to insure each load gets its required flow rate. This tedious step can take a crew of operators

many days to complete.

In this case s tudy, we will see how PIPE-FLO can be used to determine the position ofa valve to balance the system,

cutting days off the balancing process. In addition, we will see how balancing the system can provide extra system capacity

along with minimizing the system's pumping cost.

ThePiping System

In looking at the FLO-Sheet you can see the system consists of an open loop cooling system, which takes suction from the

cooling tower sump, through one of two circulating water pumps, then to three buildings . Each building has four cooling

loads requiring varying capacity. After exiting the heat exchangers, the fluid goes through a throttle valve, then a common

return line to the spray header of the cooling tower. Here the heat is released to the atmosphere.

A system expansion was recently completed at which time the loads in Building 123 were added to the system. The

circulating water pumps CCW-15 and CCW-16 were sized so one pump could s upply the entire cooling loads including the

Building 123 expansion. After 6 months of operation with Building 123 online, the circulating water pump CCW-15 tripped

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due to excess ive electrical load. To correct this problem, operations started running both cooling water pumps continually.

Since then, there have been no problems with either pump tripping on overload.

The plant's engineer at this s ite was instructed to add a third circulating water pump as a standby to improve the system

reliability. Before adding the third pump, the plant engineer wanted to get a better understanding why the existing pump

arrangement would not meet the loads as designed.

How the System Operates

The first s tep is to determine how the system is currently operating and why the system will no longer work with a single

pump.

It can be s een that the flow rate through the pump CCW-15 exceeds the pump's run out of 2,978 gpm, which is why the

motor for the single pump tripped on overload.

Next, see how the system operates with both pumps running:

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In reviewing the results, with two pumps running neither pump is in a run out condition. That explains why the pumps no

longer trip on overload with both pum ps running. Notice, however that many of the components are red, indicating the flow

rate through those components exceeds the manufacturer's lim its. This could cause a problem over time due to high fluid

velocities in the heat exchanger tubes.

After a trip to the field, the plant engineer noticed all the throttle valves were wide open. After further investigation, it was

discovered that the system was never re-balanced after Building 123 was added to the system. When the new bui lding

went online, the flow rates to the loads in the two existing buildings dropped. The operators opened the valves in these two

buildings to increase the flow rate. As each throttle valve was opened, the flow to the adjacent valves decreased. Before

long, the operators opened all the throttle valves in the system to make sure all loads received their required capacity.

Before s izing the third pump, the plant engineer decided to use the computer model to see if a balanced system was

possible us ing a single pump, as originally designed.

Calculating the Balanced System

To adjust the flow rates to the des ign values, balancing valves in each loop are throttled to increase the differential

pressure across the loop, thereby lowering the flow rate in that loop. This also causes the flow rate in the adjoining loops

to increase, resulting in increased differential pressure in these. To balance a system, all the valves mus t be throttled and

the system resul ts checked multiple times until the flow rates through each loop are at the design value. This is a tedious

and time-consuming process when done by trial and error in the field.

To accomplish this in PIPE-FLO, FCV's are ins erted into each loop and s et to the design flow rate. The program then

calculates the differential pressure across each throttle valve needed to balance the flow for each loop.

The table below s hows the des ign flow rates required for each load in the system

Load Design Flow Rate (gpm) Load Design Flow Rate (gpm)

HX-1 130 HX-7 175

HX-2 150 HX-8 100

HX-3 150 HX-9 300

HX-4 130 HX-10 140

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HX-5 700 HX-11 150

HX-6 400 HX-12 130

Regulating the flow rate through the pipelines

Now we will s et each of the FCVs to the specified flow rate.

PIPE-FLO calculates the differential pressure needed to lim it the flow rate through the FCVs to the set values. When the

system is balanced, the total flow rate through the system is 2,655 US gpm, which is less than the run out flow rate for the

pump.

As you can see, the system can run with only one CCW pump in operation. This elim inates the need to purchase and

install a third standby pump. By having only one pump operating, we also save on annual pumping costs. Finally the

system only requires 2655 gpm , which is well within a s ingle pump's operating curve, giving the plant future system

capacity as well.

Balancing the System

The final s tep in balancing the system is to insert restrictions in each pipeline. These res trictions provide the differential

pressure needed in each loop to balance the system as calculated by PIPE-FLO. The FCV restriction calculated by PIPE-

FLO can be s upplied by:

A modulating control valve controlled by a flow control circuit

A balancing orifice

The method chosen is based on the degree of control desired. Regardless of the method, PIPE-FLO can meet your needs.

For a modulating valve, you can have PIPE-FLO select a control valve to be placed into the system. You can then insert the

control valve and PIPE-FLO will modulate the valve and display the valve position based on the set point.

For a balancing orifice, you can insert an orifice instead of a FCV.You can then use the flow meter sizing feature of PIPE-

FLO. This calculates the s ize of a balancing orifice needed to provide the required pressure drop at a certain flow.

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